writeback: separate out include/linux/backing-dev-defs.h

With the planned cgroup writeback support, backing-dev related
declarations will be more widely used across block and cgroup;
unfortunately, including backing-dev.h from include/linux/blkdev.h
makes cyclic include dependency quite likely.

This patch separates out backing-dev-defs.h which only has the
essential definitions and updates blkdev.h to include it.  c files
which need access to more backing-dev details now include
backing-dev.h directly.  This takes backing-dev.h off the common
include dependency chain making it a lot easier to use it across block
and cgroup.

v2: fs/fat build failure fixed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fs/hfs/super.c b/fs/hfs/super.c
index eee7206..55c03b9 100644
--- a/fs/hfs/super.c
+++ b/fs/hfs/super.c
@@ -14,6 +14,7 @@
 
 #include <linux/module.h>
 #include <linux/blkdev.h>
+#include <linux/backing-dev.h>
 #include <linux/mount.h>
 #include <linux/init.h>
 #include <linux/nls.h>